A solid-state synthetic strategy toward nickel-based bimetallic interstitial compounds (MNi₃C_x, M = Zn, In, Ga)

Bimetallic interstitial compounds with unique geometric properties have attracted increasing attention in energy-related fields and diverse chemical transformations. Current synthesis of these compounds generally involves at least one wet-chemistry step with the use of various solvents to prepare the bimetallic precursors, and no universal protocols for different compositions are yet available. Herein, a novel synthetic strategy toward a platform of nickel-based bimetallic interstitial compounds with the formula MNi3Cx, M = Zn, In, and Ga, was developed based on a straightforward solid-state transformation, i.e., simply annealing the hydroxides of the respective metals in the presence of different carbon precursors (cyanamide, dicyandiamide, melamine, and urea) in a hydrogen stream. The key process parameters influencing the compositions of the final products are studied and the formation mechanism is discussed based on advanced characterization techniques. Powder X-ray diffraction reveals MNi3Cx as a single phase and electron microscopy shows that the MNi3Cx particles are covered with N-doped carbon shells. Extrapolation to other bimetallic interstitial compounds failed when following the above protocol, and the successful examples are linked to the formation of the corresponding bimetallic alloys in the absence of carbon precursors. When evaluated for the selective hydrogenation of dimethyl oxalate, both InNi3C0.5 and ZnNi3C0.7 show comparable high activity. While ZnNi3C0.7 delivers the highest selectivity for methyl glycolate, tunable methyl glycolate and ethylene glycol are formed on InNi3C0.5. In general, this facile solvent-free strategy affords an interesting scaffold to fabricate more advanced multi-metallic interstitial compounds with broad applications.